A typical molten metal facility includes a furnace with a pump for moving molten metal. During the processing of molten metals, such as aluminum, the molten metal is normally circulated through the furnace by a centrifugal pump to equalize the temperature of the molten bath and to transfer the molten metal out of the pump. These pumps contain a rotating impeller that draws in and accelerates the molten metal creating a laminar-type flow within the furnace.
The impeller of the present invention is particularly well suited to be used in molten aluminum and molten zinc pumps. In fact, throughout the specification, numerous references will be made to the use of the impeller in molten aluminum pumps, and certain prior art molten aluminum pumps will be discussed. However, it should be realized that the invention can be used in any pump utilized in refining or casting molten metals.
In the processing of molten metals, it is often necessary to move molten metal from one place to another. When it is desired to remove molten metal from a vessel, a so called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so called gas injection pump is used. In each of these types of pumps, a rotatable impeller is disposed within a pumping chamber in a vessel containing the molten metal. Rotation of the impeller within the pumping chamber draws in molten metal and expels it in a direction governed by the design of the pumping chamber.
In most centrifugal pumps, the pumping chamber is formed in a base housing which is suspended within the molten metal by support posts or other means. The impeller is supported for rotation in the base housing by means of a rotatable shaft connected to a drive motor located atop a platform which is also supported by the posts.
Molten metal pump designers are generally concerned with efficiency, effectiveness and longevity. For a given diameter impeller, efficiency is defined by the work output of the pump divided by the work input of the motor. An equally important quality of effectiveness is defined as molten metal flow per impeller revolutions per minute. Generally speaking, improved efficiency of the metal flow is achieved by making the pump exit velocity as high as necessary to efficiently discharge the metal so as to penetrate the metal pool outside the pump, while maintaining the pump as small as possible.
Typically, conventional impellers have much larger outlet openings than the inlet opening's size due to the impeller's diametral increase from the radially inward inlet to the outwardly located outlet, this increase in opening size normally results in a dramatic reduction in the radial velocity component of these prior impellers.
My present invention improves efficiency and flow by increasing the total velocity of the fluid exiting the impeller of a centrifugal pump. This increase in output velocity of the pumped fluid is achieved by curving the impeller passages towards the direction of rotation of the impeller. The curved passages maintain a specially configured cross-sectional area and shape through the length of the passage to ensure that there is no significant loss in the radial velocity of the fluid (created by the rotation of the impeller) other than inherent losses attributed to changing from axial flow to radial flow as the fluid travels through the passage. The forwardly directed passages in combination with the size of the passages results in the re-direction of the majority of the radial velocity component into the tangential direction, thereby increasing the total pump outlet velocity and assuring higher flows at equal volute cross-sectional areas compared to traditional impeller designs.
The present invention increases flow approximately 25% over my prior U.S. Pat. No. 7,326,028 entitled HIGH FLOW/DUAL INDUCER/HIGH EFFICIENCY IMPELLER FOR LIQUID APPLICATIONS INCLUDING MOLTEN METAL, which is incorporated herein in its entirety, which provided flow rates of 2000 gallons of molten aluminum per minute at 300 rpm for a 16 inch diameter impeller. The present invention achieves approximately 2500 gal/min at 300 rpm using only a 14 inch diameter impeller. Further my prior impeller produced head coefficients (k) between 0.52-0.54, while I am now able to achieve approximately 0.55-0.57 with my present invention.
Another troublesome aspect of molten metal pump operation is the degradation of the impeller. Moreover, to operate in a high temperature, abrasive molten metal environment, a refractory or graphite material is used from which to construct the impeller because of their inert qualities. However, these materials are also prone to degradation when exposed to particles entrained in the molten metal. More specifically, the molten metal may include pieces of the refractory lining of the molten metal furnace, undesirable material from the metal feed stock and occlusions which develop via chemical reaction or metallurgical combination, all of which can cause damage to an impeller and pump housing if passed therethrough.
My present centrifugal pump impeller has fluid passages that have a cross-sectional area and shape that absolutely gradually increases from the inlet openings all the way to the outlet opening. This progressive area and shape ensures that any particulate matter (e.g., dross) that finds its way into the impeller will pass through the impeller and will not become lodged in the rotating impeller, thereby avoiding a catastrophic failure of the pump.
The novel impeller has a generally cylindrical shape and is formed of a refractory material such as graphite or a ceramic such as silicon nitride silicon carbide. The cylindrical piece includes a hub surrounding a cavity in its upper face suitable to accommodate a shaft. The shaft, in turn, is joined to a motor to achieve rotation of the impeller. The periphery of the upper face is machined to include a plurality of passages which extend downwardly and outwardly from the upper face to the sides of the cylindrical impeller.
Importantly, each of the impeller passages is curved toward the direction of the impeller's rotation and has a gradually increasing cross-sectional area and shape. Maintaining this type of passage and curving the passage toward the direction of rotation re-directs the radial velocity of the flowing liquid to add its velocity to the tangential velocity imparted on the flow by the rotating shaft-impeller assembly. In one preferred embodiment, five passages are formed and provide a large inlet fluid volume area.
Further, the passages are formed such that they provide a “tunnel” at the upper face of the impeller after a cover plate is provided or when the impeller is ceramic casted (having an integral “top plate” formed thereon), which effectively provides entrainment of any particular particles (that are smaller than the inlet openings) entering the impeller and prevents lodging/jamming between the rotating impeller body and the pump housing. In this manner, any inclusions or scrap contained in the molten metal which is small enough to enter this zone of the passage will of necessity be sized such that it can exit the impeller.
It is an advantage of the present invention to provide a centrifugal pump impeller system for pumping fluid, including molten metal, comprising an impeller adapted for rotating about an axis in a certain pumping direction of rotation. The impeller comprising a circular and generally flat base and a plurality of vanes mounted to the base. The vanes extending radially from a radially inward portion of the base to an outer-most edge of the base, each vane having a concave leading wall and a convex trailing wall, the trailing wall of each vane cooperating with the leading wall of an adjacent vane to define the next curved passage. Wherein the trailing wall of each vane is complementary in shape to the adjacent leading wall, such that the passage has a gradually increasing cross-sectional area from a radially axial inward inlet to a radially outward outlet, and wherein the impeller is rotatable about a central axis such that fluid flowing through each passage follows the curved leading wall into the same general direction as the pumping direction of rotation. The curved passage walls adding a portion of the radial velocity of the fluid to the tangential velocity of the flow to increase the total velocity of the fluid exiting the impeller.
It is another advantage of the present invention to receive the fluid exiting from the impeller which would ordinarily drag against the outer surface of the impeller into a cavity formed in the outer surface of each vane. Each cavity traps and guides this fluid along a curved wall which redirects the fluid into the same general direction as the pumping direction of rotation, thereby adding a portion of the redirected fluid's radial velocity to the tangential velocity of the flow to increase the total velocity of the fluid exiting the impeller.
These and other objects, features and advantages of the present invention will become apparent from the following description when viewed in accordance with the accompanying drawings and appended claims.